Devices for imaging body cavities or passages in vivo are known in the art and include endoscopes and autonomous encapsulated cameras. Endoscopes are flexible or rigid tubes that pass into the body through an orifice or surgical opening, typically into the esophagus via the mouth or into the colon via the rectum. An image is formed at the distal end using a lens and transmitted to the proximal end, outside the body, either by a lens-relay system or by a coherent fiber-optic bundle. A conceptually similar instrument might record an image electronically at the distal end, for example using a CCD or CMOS array, and transfer the image data as an electrical signal to the proximal end through a cable. Endoscopes allow a physician control over the field of view and are well-accepted diagnostic tools. However, they do have a number of limitations, present risks to the patient, are invasive and uncomfortable for the patient, and their cost restricts their application as routine health-screening tools.
An alternative in vivo image sensor that addresses many of these problems is capsule endoscope. A camera is housed in a swallowable capsule, along with a radio transmitter for transmitting data, primarily comprising images recorded by the digital camera, to a base-station receiver or transceiver and data recorder outside the body. The capsule may also include a radio receiver for receiving instructions or other data from a base-station transmitter. Instead of radio-frequency transmission, lower-frequency electromagnetic signals may be used. Power may be supplied inductively from an external inductor to an internal inductor within the capsule or from a battery within the capsule.
An autonomous capsule camera system with on-board data storage was disclosed in the U.S. patent application Ser. No. 11/533,304, entitled “In Vivo Autonomous Camera with On-Board Data Storage or Digital Wireless Transmission in Regulatory Approved Band,” filed on Sep. 19, 2006. This application describes a motion detection that is conducted using a portion of each image, the portion being stored in a partial frame buffer, a metric for measuring the degree of motion between a current sub-image and a previous sub-image is used to select either Skip Mode (or Conservation Mode) or Capture Mode. U.S. Pat. No. 7,940,973, entitled “Capture Control for in vivo Camera”, issued on May 10, 2011 further discloses variations of capture control. In one embodiment, a full-size full-resolution image is capture when Capture Mode is selected. When Skip Mode (Conservation Mode) is selected, the capsule camera exercises power conservation such as reducing luminous energy emitted from a light source, disregarding the input image, and/or capturing the input image in low resolution or low quality.
While the applications mentioned above use motion detection and motion estimation to eliminate some unnecessary image capture and conserved the precious on-board storage and battery power, it is desirable to further reduce storage requirement by using high efficiency compression. The need for high efficiency compression becomes more apparent when the required image resolution continues to grow. Accordingly a capsule camera that uses motion evaluation to reduce system power and unnecessary image capture, and uses motion-compensated video compression to encode the captured images is disclosed.